There have been made many proposals on solid catalyst components containing a titanium halides, a magnesium compound and an electron donor compound as essential components, and polymerization of olefins in the presence of a catalyst comprising such a solid catalyst component, an organic aluminium compound and a silicone compound.
Solid catalyst components comprising a dialkoxymagnesium and titanium tetrachloride as primary starting materials, and catalysts for polymerization of olefins which comprise such a solid catalyst component, an organic aluminium compound and a silicone compound are also known as described in, for example, JP-A-63-3010 (the term "JP-A" as used herein means an unexamined published Japanese patent application), JP-A-1-221405, JP-A-1-315406, JP-A-3-227309, JP-A-3-70711, and JP-A-4-8709.
Further, solid catalyst components containing an aluminium halide, a magnesium compound and a titanium halide as essential components, and catalysts containing such a solid catalyst component, an organic aluminium compound and as the third component, an organic acid ester or a silicone compound are known. For example, JP-A-55-161807 discloses a catalyst comprising (i) a composition obtained by pulverizing magnesium chloride, an organic acid ester, a halogenated hydrocarbon and an aluminium halide together and then carrying out heat treatment with titanium tetrachloride, (ii) an organic aluminium compound, and (iii) an organic acid ester; and JP-A-61-31402 discloses a catalyst comprising (i) a solid catalyst component obtained by reacting a reaction product of an aluminium halide and a silicone compound with a magnesium compound and then reacting with a titanium halide and an ester of phthalic acid, (ii) an organic aluminium compound and (iii) a silicone compound.
Furthermore, solid catalyst components containing an alkoxyaluminium compound, a magnesium compound and a titanium halide as essential components, and catalysts for polymerization of olefins which contain such a solid catalyst component, an organic aluminium compound and as the third component, an organic acid ester or a silicone compound are also known. For example, JP-A-57-145104 discloses a catalyst component obtained by pulverizing magnesium chloride, an organic acid ester and an alkoxyaluminium compound together and then carrying out heat treatment with titanium tetrachloride; and JP-A-1-245002 discloses a catalyst comprising (i) a solid catalyst component obtained by bringing diethoxymagnesium into contact with titanium tetrachloride, followed by addition of an trialkoxyaluminium and then reacting with phthalic dichloride, (ii) an organic aluminium compound, and (iii) an epoxy-p-menthane compound.
These solid catalyst components and catalysts as described above have been developed as a result of studies for attaining high catalytic activity in polymerization of propylene so that the amount of the solid catalyst component can be minimized and the step of removing a catalyst residue (e.g., chlorine and titanium) remaining in a resulting polymer can be omitted, for improving the yield of a stereoregular polymer, or for improving durability of the catalytic activity for polymerization, and they provide good results for the respective objects. However, none of these catalysts can produce stereoregular polymers having a density of from 0.900 to 0.906 g/ml without lowering the yield of stereoregular polymers insoluble in a polymerization solvent as used in polymerization of olefins, particularly polymerization of propylene, according to the slurry method.
In the case where olefins, particularly propylene are polymerized by the slurry method in the presence of the aforesaid highly active catalyst, the resulting polymer has a high stereoregularity and is obtained in a high yield as compared with the case where a catalyst comprising a conventional titanium trichloride type solid catalyst component, an organic aluminium compound and an electron donor compound is used. However, the density of the resulting polymer tends to be higher than 0.906 g/ml, giving rise to various problems, e.g., breaking in rapid rolling rate and deterioration in transparency of a film product molded by a BOPP molding.
It has been known that the density of the resulting polymer can be controlled to some extents by lowering the polymerization temperature or introducing a small amount of ethylene as a comonomer into the polymerization system of olefins (particularly propylene) in the presence of the aforesaid highly active catalyst. In case of the slurry method, however, an undesirable phenomenon occurs that a low molecular weight polymer which is soluble in a polymerization solvent is formed at a high rate, and in the polymerization of propylene or copolymerization of propylene and ethylene, an atactic polypropylene which has an extremely poor stereoregularity is generated at a high rate. The atacticity can be evaluated in terms of the content of soluble portions of the resulting polymer or copolymer in a polymerization solvent, which is hereafter referred to "RDS" (the content of reactor diluent solubles).
Increase of the RDS in the slurry polymerization gives rise to problems with respect to the production cost of a polymer and the stability in operation because a reactor and a pipeline are stained, and an extraction step is required after separation of the particles of a resulting polymer from a polymerization solvent. Further, generation of fine powders contained in the resulting polymer, particularly those having a particle size of 100 micron or less, tend to cause clogging in a pipeline in the polymerization process, and other problems in the step of separation and drying of the polymer.